In fill-level measuring devices according to the pulse transit-time method, a reflection that has been generated internally in a microwave module is used as a reference for transit time measuring.
The stability of this internally generated reflection as far as amplitude, shape and location are concerned is decisive to the accuracy of the measuring device, because any change may result in a shift in the reference (zero point).
In hitherto known fill-level measuring devices the reference pulse is obtained from a mixture of different reflections. These individual reflections arise, for example, by way of the antenna input, the plug-and-socket connections at the microwave module, the transition from the printed circuit board to coaxial lines or waveguides, as well as by way of leakage signals in the transmitter-receiver filter circuit (e.g. in the directional coupler or in the circulator). Furthermore, by superposition with very large echoes of a product contained in a container, the shape of the reference pulse may change to such an extent in the region of the antenna that inaccuracies may occur.
By inserting lines between different circuit components, wherein the length of the lines corresponds at least to the resolution of the fill level radar, different reflections may be better separated from each other.
In order to reduce any influencing as a result of echoes at short range, a line with the length of a transmission pulse (corresponds approximately to the resolution of the sensor) can be used between the transmitter-receiver filter circuit and the transition from the microstrip line to the coaxial line. Likewise, a piece of coaxial line can be used between this transition and the waveguide coupling.
However, the influence which the remaining reflections have on the reference pulse may remain in this arrangement. If, for example, the insulation of the transmitter-receiver filter circuit changes over temperature, this also may affect the reference pulse and thus the measured value.
A further option for obtaining the reference pulse may consist of coupling out part of the signal already significantly earlier in the circuit and, by way of a directional coupler, feeding said part of the signal into the mixer. However, generally speaking, this may require delaying the transmission signal so that the undesired reflections can reliably be separated from the purposefully obtained reference signal. However, in this arrangement the transit time line may have relatively large attenuation, which may have to be compensated for by means of an amplifier. This may result in increased current consumption by the microwave module.